An air filter can basically have a filter housing, in which a filter element is arranged and thereby separates a raw side from a clean side in the filter housing. The filter element here can have an annular filter body of a folded filter material. The folded or pleated filter material here has two end folds or end sections, which are adjacent in the circumferential direction of the annular filter body and are fastened to each other. For fastening the two end folds to each other, it is basically possible to glue the end folds to each other or to weld them to each other or to fasten them to each other by means of at least one clip.
Owing to the fastening of the end folds to each other, the filter body has in the region of the end folds which are fastened to each other a different, namely an increased, through-flow resistance than in the other regions, because the end folds which are fastened to each other lead there to a certain sealing of the filter material. In this respect, a non-homogeneous distribution of the through-flow resistance is produced in circumferential direction of the filter body.
In particular applications, a symmetrical flow pattern downstream of the filter element can be significant. For example, in a fresh air system downstream of the filter element an air-flow sensor, in particular in the form of a hot film sensor, can be arranged in order to detect for example the currently flowing air quantity for an engine control. If for any reason it is necessary to position this air-flow sensor relatively close to the filter element, the relative rotational position of the filter element in the filter housing can have an influence on the measurement of the air-flow sensor. In order to reduce the influence of the rotational position of the filter element on the air-flow measurement, it is possible to calibrate the air-flow sensor only with an installed filter element. This may still be able to be realized with justifiable expenditure on the initial equipping of a vehicle, but not for the case where the filter element is exchanged within an inspection. The newly inserted filter element then regularly has a different relative rotational position with respect to the filter housing and therefore also with respect to the air-flow sensor, which makes a renewed calibration of the air-flow sensor necessary. The expenditure for this is comparatively high and can, in particular, exceed the material costs for the new filter element and the labour costs for the exchange of the filter element.